Feature flags separate deployment from release. You deploy code with features hidden behind flags, then enable them independently.
Benefits:
- Test in production with flags off
- Gradual rollout to user segments
- Kill switch for broken features
- A/B testing
Tools: LaunchDarkly, Split, Unleash, ConfigCat
Progressive delivery pattern:
Deploy with flag off
Enable for internal users
Enable for % of traffic
Monitor and expand
Remove flag when stable
Interview tip: Feature flags add complexity (flag debt). Mention cleanup processes when discussing them.